The present invention relates to the field of video communications, and specifically to an apparatus and method for achieving a three-dimensional video image using a refracticular screen surface.
It has long been a desired object in the field of video communications to achieve a commercially practical three-dimensional viewing system, especially for in-home television viewing. While numerous solutions have been proposed, heretofore none have been successful in gaining widespread acceptance. Two basic problems exist with known methods of achieving three-dimensional video images. First, most systems require the viewer to use some type of extraneous device to polarize an image being viewed, such as glasses with colored lenses like those distributed for viewing 3-D films in movie theaters. A second problem is that most systems require complex manipulation of the video signal, either on the transmitting end, the receiving end, or both. With such systems, three-dimensional viewing requires a significant financial expenditure and/or significant modification of existing equipment.
In a modern television system, a television camera tube typically includes an electron gun which fires a beam of electrons toward a light-sensitive surface. To obtain an image, the electron beam scans the light-sensitive surface horizontally and vertically in a series of 525 horizontal scan lines disposed one above the other. The horizontal scan occurs at a uniform speed from left to right, with a much faster return to the left. Similarly, the vertical scan occurs at a uniform speed from top to bottom, with a much faster retrace to the top of the screen. The light-sensitive surface produces an electrical output signal when the electron beam strikes it, which is then input to a video amplifier. Since the magnitude of the voltage produced is proportional to the light intensity at that point on the light-sensitive surface, the video information signal generated by the video amplifier furnishes an electrical representation of the image viewed by the camera. Prior to being transmitted to a receiver, the video information signal is mixed with a synchronization signal that will enable the receiver to correctly reproduce the scanning pattern used by the camera to generate the signal.
At the receiver, the video information signal drives a grid for an electron gun in a picture tube, such as a cathode ray tube (CRT). The electron gun causes the CRT to display an image by varying the intensity of a beam of electrons according to the amplitude of the received video signal as the beam is focused onto a fluorescent screen at the face of the CRT. A peak in the video signal will result in a very bright spot, while a lower amplitude in the video signal will result in a darker spot. While the electron beam intensity is varied according to the amplitude of the received video signal, the electron beam is simultaneously scanned across the face of the CRT in a pattern duplicating the left-to-right, top-to-bottom motion of the beam that scanned the face of the TV camera tube. The two electron beams are synchronized using the aforementioned synchronization signal; thus, the image formed on the CRT consists of a pattern of varying-intensity light traced sequentially in the same fashion as the image processed by the television camera.
Since the image is formed sequentially, the beginning of the image must remain in view while the rest of the image is being formed so that a viewer perceives a complete picture. To this end, the fluorescent screen of the picture tube and the human eye work together by retaining the spot images on the tube long enough for the eye to see a complete image. In most television systems, this laying down of the sequential screen image happens at a rate of 30 complete, 525-line frames each second. This scan rate, however, has been found to present some problems because the viewer may sometimes notice the picture flickering due to the minute delays between the formation of one image and the next. Modern television systems therefore employ "interlaced scanning" of picture images to eliminate this phenomenon.
With interlaced scanning, a 525-line frame is treated as two interleaved fields of horizontal scan lines, wherein each field is scanned 60 times per second. That is, a field consisting of all of the odd-numbered lines in a 525-line frame are scanned in 1/60 second, followed by a second field consisting of all of the even-numbered lines. Due to the rapid rate at which the alternate fields are painted on the screen, a viewer will not perceive each field as a broken image. Moreover, the human eye is unable to detect an image flicker in the 1/60 second that elapses between painting of the top line and the bottom line of the image. By interleaving the respective odd and even fields, a complete 525-line frame is formed 30 times each second.
The three-dimensional video system of the present invention exploits the human brain's ability to perceive three-dimensional data by triangulating the images provided from the slightly-different perspectives of the left and right eyes. The human brain will inherently attempt to convert a two-dimensional subject, such as a television picture, into a three-dimensional image if given the opportunity to receive slightly different perspectives from each eye. However, two factors associated with standard television pictures prevent the brain from being successful. First and foremost, a video signal generally represents only a single perspective, as provided by a standard, single-lens video camera. Second, even if the video signal were to include two different perspectives, a standard video screen is incapable of directing the different perspectives to the appropriate eyes to enable the brain to triangulate the information.
An approach to three-dimensional video illustrating some of the problems of prior art systems is described in U.S. Pat. No. 4,963,959 to Drewlo, entitled "Three-Dimensional Cathode Ray Tube Display." Drewlo uses two modified television cameras mounted side-by-side to obtain video images from two different perspectives. Unlike conventional cameras, the electron guns of the Drewlo cameras sweep a scene in a top-to-bottom, left-to-right pattern, thereby providing a vertically-oriented scan. The signals from the two cameras are then combined by a signal mixer into a composite signal before being transmitted to a receiver. At the receiver, the composite signal is split into its two constituent signals and either fed to two separate electron guns or a single electron gun which time division multiplexes the beams. In either case, the separated right and left beams are passed through a Fresnel lens attached to the inner surface of the viewing screen. The Fresnel lens includes a series of vertical grooves which define a plurality of contiguous, closely-spaced and vertically-oriented prisms which provide optical isolation in the horizontal plane, thereby directing independent video streams towards each eye. In order to align the sweep of the picture tube's electron beam with these vertical prisms, deflection circuitry associated with the neck of the cathode ray tube must be rotated so that the face of the screen is swept by the electron beam in vertical strokes, rather than horizontal strokes. As can be seen, the Drewlo approach undesirably requires a significant modification of the receiver.